The long-term goal of this project is to understand the process of speciation through genetic analysis of its products: closely-related species. Our method uses classical genetic techniques to reveal the numbers, locations, and effects of genes causing sexual isolation and morphological differences between Drosophila species, and genes causing sterility and inviability of their hybrids. A collection of such analyses, conducted in three different Drosophila groups, will reveal patterns that address the following questions: 1. Are reproductive isolation and morphological differences between species due to only a few genes of large effect, or do they have a more polygenic basis? 2. Where are the genes causing reproductive isolation? Are they located in similar regions among different species pairs of a group, implying genetic "hotspots" of speciation? When they cause sexual isolation (mating discrimination), which aspects of courtship are affected? 3. Are the genes causing sexual isolation concentrated on the sex chromosomes, like those causing postzygotic isolation? 4. Is the genetic basis of sexual isolation similar in males and in females? 5. Are there consistent patterns to the genetics of reproductive isolation that hold across different groups? 6. What is the genetic basis of maternal effects that cause sterility of hybrids? 7. Can genetic drift contribute to speciation in small colonizing populations? Our data will not only help resolve current controversies about the genetics of speciation, but also motivate new theories. The results may be relevant to understanding the origin of species in other animals, including humans.